The present invention relates to a DC-DC converter, and more particularly, to a controller and a control method for a DC-DC converter.
Many portable electronics devices are being used in recent years. A portable device uses a battery as a drive power supply. The output voltage of the battery decreases as the device is used or as the battery is discharged. An electronics device includes a DC-DC converter for converting the voltage of its battery to a constant voltage. The conversion efficiency of the DC-DC converter affects the operation time of the electronics device that is operated by the battery. The operation of the electronics device may require a different amount of current. In view of this, the electronics device requires a DC-DC converter having high conversion efficiency for any current amount and a controller for such a DC-DC converter.
In the prior art, a portable electronics device uses a switching DC-DC converter (hereafter referred to as a “switching regulator”), which is compact and has high conversion efficiency. The switching regulator is a pulse width modulation (PWM) regulator. The PWM regulator keeps the output voltage substantially constant by controlling the pulse width of a pulse signal for driving its main switching transistor in accordance with the output voltage or the output current.
To extend the life of the battery and extend the operation time of the electronics device that is operated using the battery, the electronics device may reduce its consumption current. In such a low load state, the conversion efficiency of the PWM switching regulator is extremely low. The reasons for this will be described below.
The power loss of the switching regulator includes loss that is dependent on the output current of the regulator and a fixed loss that is not dependent on the output current of the regulator. The loss dependent on the output current includes power loss that is generated by the ON-resistance of transistors in the regulator. The fixed loss that is not dependent on the output current may include power loss that is generated when a main switching transistor of the regulator is activated and inactivated, and a loss that is generated by power consumption of the regulator itself. When the output current increases (the difference between the input current and the output current becoming small), the loss dependent on the output current increases accordingly but the fixed loss remains unchanged. In this case, the conversion efficiency of the switching regulator is not low as a whole. However, when the output current decreases (the difference between the input current and the output current becoming large), the ratio of the fixed loss to the loss dependent on the output current increases. In this case, the conversion efficiency of the switching regulator decreases as shown in FIG. 1.
The fixed loss of the switching regulator is proportional to the switching frequency for activating and inactivating the main switching transistor. Thus, the power loss may be reduced by lowering the switching frequency. However, ripple of the output voltage increases when the switching frequency is lowered.
Japanese Laid-Open Patent Publication No. 2003-9515 describes a power supply system that includes a switching regulator and a linear regulator. The system switches between the two regulators in accordance with the load current. The loss of the linear regulator is dependent on the difference between the input voltage and the output voltage. Further, the loss of the linear regulator is not dependent on the difference between the input current and the output current. The linear regulator has high conversion efficiency in a low load state, as shown in FIG. 2.
As shown in FIG. 3, the DC-DC converter realizes high conversion efficiency irrespective of the output current when the DC-DC converter is switched between the switching regulator and the linear regulator at a load current I1 with which the efficiency of the switching regulator and the efficiency of the linear regulator substantially coincide with each other.